This invention relates generally to liquid chromatography and more specifically relates to a solvent supply system for use in high performance column liquid chromatography.
Chromatography is a separation method wherein a mixture of components (called the "sample" or "sample mixture") is placed as a zone at one end of a system containing a stationary phase and a mobile phase. Each component of the sample distributes itself in dynamic equilibrium between the two phases in a ratio characteristic of that component. As a result, the flowing mobile phase causes each individual component zone to migrate at a characteristic rate, and the zones become separated after a period of time.
There are various types of chromatography, e.g., liquid chromatography, gas chromotography, thin-layer chromatography, etc. The major differences between these various chromatographic methods is the physical state of the mobile phase (gas or liquid), and the manner in which the stationary phase is supported, e.g., coated on an inert granular material packed in a tube, coated on an inner wall surface, etc. In each method, the separation mechanism is essentially the same, i.e., distribution of the sample components between a mobile phase and a stationary phase. When the method is used for chemical analysis, a detector is commonly placed at the far end of the system, so as to monitor the passage of the component zones as they emerge from the system. The signal from the detector is displayed on a recording device such as a strip chart recorder, and the record indicates both qualitative and quantitative information regarding the components of the sample.
It is often desirable for a chromatographic system to provide high resolution (a large degree of component separation with narrow zones), evenly spaced component zones, rapid separation, and a satisfactory record from a very small sample. The behavior of the system described in these terms may be called the "performance" of the system. It is well known in the chromatography art to improve system performance by changing one of the following system variables during the course of the analysis: temperature, chemical composition of the mobile phase, and flow rate of the mobile phase. For example, in gas chromatography the temperature of the system is often varied as a preselected function of time. This technique is known as "temperature programming", and it improves the performance of the system, especially with samples containing components which boil over a wide temperature range. Analagous to temperature programming in gas chomatography, is the use of "gradient elution" in liquid chomatography. Gradient elution refers to changing the chemical composition of the mobile phase (also called the "eluent" or "eluting solvent") as a function of time, thereby improving the performance of the system, especially with samples containing components which vary widely in chemical properties. The net effect of gradient elution is to shorten the retention time of compounds strongly retained on columns without sacrifice in separation of early eluting compounds. Further details regarding the fundamentals of gradient elution techniques may be found in numerous sources in the prior art, as, for example, in the publication by L. R. Snyder appearing in Chromatography Review 7,1 (1965).
A central concern pertinent to liquid chromatography apparatus of the type considered herein, is one of providing a proper flow of solvent to and through the chromatography column. Thus in the past, numerous and varied approaches have been utilized for supplying solvents to high performance liquid chromatography columns. A key requirement in this connection is one of providing a relatively non-pulsating, i.e., a constant flow of solvent -- in that the LC detector is sensitive to flow variations, and can provide erroneous readings and exhibit excessive noise in the presence of pulsing flow. Various approaches have been utilized in the past in order to enable such result, but in general, the prior art methodology directed at such end has involved highly expensive and overly complex mechanisms. Thus, in a typical example wherein a system is intended for operation in a gradient elution mode, i.e., by use of two distinct solvents, a dual pump arrangement may be utilized. Such arrangement requires two distinct pumps, including separate means for driving each of the pumps, which thus requires separate speed controls, etc.
In principle, it would seem that the cited problems arising in connection with the solvent pumping systems of the prior art, might be overcome by use of a single cylinder arrangement in cooperation with a relatively small displacement volume reciprocating piston. A principal deterrent to the use of this arrangement, however, has been the fact that the ensuing flow will, by its nature, be pulsating -- particularly at low flow rates. Further, the nature of the pulses present in the flow is such that they are not easily removed by filtering and the presence of such pulses can sharply limit the efficiency of the detector system. It should be understood in the foregoing connection that the word "piston" as used in this specification is intended to include both pistons where the seal remains fixed in relative position to the moving member and plungers where the seal is fixed with respect to the stationary cylinder.
It has in the foregoing connection, been long recognized that the aspect of the reciprocating pump which is principally responsible for an unacceptable pulsating flow is the fact that when the pump piston is driven by a simple crank shaft mechanism, the axial displacement of the piston as a function of time is sinusoidal. This implies the presence of equal time spaced pressure (or liquid pumping) pulses, alternating with fill periods of duration equal to the pressure pulse duration. In an effort to overcome this pattern, it has been proposed to drive the piston through suitably shaped cams. Pursuant to such approach these serve to alter the time displacement function of the pump piston so as to foreshorten the fill portion of the cycle in comparison to the pumping portion, and in some instances render the movement during pumping relatively linear in nature, i.e., the displacement is linear as a function of time. This sort of arrangement does have the advantage of changing the form of the pulsating pattern so as to diminish the pulsing and render filtering of the remaining pulses more feasible. However, the approach is less than satisfactory in a most important respect. In particular, the cam represents a fixed pattern, and thus provides a fixed relationship or ratio between the fill and pumping portions of the pump cycle. And yet, in many instances it is desired to have a capability for operation over various flow rates -- which indeed can vary very widely. If, however, the flow rate is increased by merely increasing the rate of cam rotation, then the fill portion of the cycle becomes successively shortened -- and can reach a point where insufficient feed time is available leading to cavitation and other problems.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide high performance liquid chromatography apparatus, which utilizes a relatively inexpensive reciprocating pump, and which yet provides a highly uniform, relatively non-pulsating flow through the chromatography column associated therewith.
It is a further object of the present invention to provide high performance, high pressure liquid chromatography apparatus, wherein the relatively inexpensive pump associated therewith operates in a reciprocating mode, and is capable of delivering uniform flow of solvent throughout a wide range of pump flow rates.
It is a still further object of the present invention, to provide high performance, high pressure chromatography apparatus, incorporating a relatively simple, relatively inexpensive reciprocating pump and additional elements which in cooperation with the pump provide highly non-pulsating uniform flow over a wide flow range; and wherein elements associated with the pump control enable simple and accurate control of solvent ratios when the apparatus is utilized with a plurality of solvents, i.e., in a gradient elution mode of operation.
It is a yet further object of the present invention, to provide a chromatography system of the foregoing high-pressure high performance type wherein the proportioning valves or similar elements utilized to provide a desired ratio between distinct solvents operate in a simple complementary fashion during a selected portion of the pump cycle, and function at the low pressure inlet side of the said pump.
It is a still further object of the present invention, to provide a chromatography system of the foregoing type, which includes filtering features at the outlet check valve for the pump portion of such system, which enables highly effective filtering of particulate matter which might otherwise impair the operation of the check valve or valves.
An additional object of the present invention is to provide a cannister type in-line dampening device, which while serving very effectively to diminish or remove pulses which may remain following the outlet valve of the system pump nevertheless works with very limited volumes of the flowing liquid, thereby facilitating fast changes in solvent composition, and not impairing purging.